A common power supply operating directly off AC line power is the switching type voltage regulator often referred to as the off-line switcher. Its widespread use is due in part to its high efficiency, precision of regulation, and fast response. A disadvantage of this type power supply is its low input impedance which induces a large in-rush current upon initial turn on or power. This is particularly critical in applications where groups of off-line switchers are connected in parallel to an AC line voltage source. To avoid tripping circuit breakers, it is often necessary to turn on the paralleled off-line switchers in controlled sequences rather than simultaneously. This requires a cumbersome arrangement for start up.
The more traditional series dissipative type voltage regulator avoids the initial turn on problem described since it does not draw large currents at turn on. However, the many disadvantages of the series dissipation regulators such as low efficiency, high thermal dissipation and low power capabilities, limits its use in off-line applications.